1. Field of the Invention
This invention relates to an optical disk apparatus, and more particularly to an optical head technology whereby plural classifications of optical disks are reproduced.
2. Description of Related Art
As commercial optical disks of a current size of 120 mm, there stand abreast a variety of standards such as DVD-ROM, DVD-RAM, DVD-R, DVD-RW, CD-R, and CD-RW in addition to the conventional music CD and CD-ROM according to the difference in recording density, capability of recording, erase, and rewriting, reliability, and distinction of application. A problem has been pointed out that these existing plural standards introduce confusions among common users, and recently optical disk apparatuses in conformity to a standard called “DVD Multi,” indicating capability of recording and reproducing all of these optical disks, have been released as commercial products.
As one of differences among the above-mentioned plural classifications of optical disks, first there can be enumerated a difference in wavelength between CD systems and DVD systems. In the beginning of release of DVD, to achieve compatibility in reproducing CDs on DVD apparatuses, the apparatus used a red semiconductor laser expressly meant for DVD. However, since CD-R in which reproduction wavelength is limited to the same infrared wavelength as the conventional CD has become popular rapidly, such an apparatus that has both the red semiconductor laser and the infrared semiconductor laser in it and uses the infrared semiconductor laser for the CD systems has become established generally.
As other differences, there can be enumerated differences in shapes of a pit sequence and of a guide groove used for tracking (hereinafter, referred to as “guide groove” as a generic term indicating the both) and a difference in the tracking system associated with those differences. For the tracking system, mainly CD-ROM uses the twin beam method; CD-R, RW, DVD-R, RW use the differential push-pull method; DVD-ROM uses the differential phase detection method; and DVD-RAM uses the push-pull method that uses a polarization-dependent diffraction grating.
The push-pull method is a method in which imbalance of a reflected light intensity distribution generated on a pupil plane of the objective lens resulting from diffraction of light by the guide groove and an interference effect between beams of the diffracted light. The imbalance of the intensity distribution on the pupil plane can be obtained by detecting the spots by two-segment detectors on planes shifted somewhat from the focal plane, on which normally the detector is placed, and obtaining a differential signal between detected signals. This is used as a tracking error signal. Since the imbalance of the intensity distribution is caused by the interference effect between beams of the diffracted light, the detection sensitivity of the tracking error by this method becomes highest when the guide groove depth is equal to an optical depth of ⅛ wavelength and the guide groove width is one-half the guide groove pitch—here, the optical depth is assumed as a depth reduced to ½ wavelength or less where the diffraction and interference effects at that depth become equivalent to those of the original (not reduced) depth using a wavelength in the substrate of the optical disk as a reference. Further, at the optical depth of ¼ wavelength, theoretically the tracking error signal amplitude by the push-pull method becomes zero. With respect to the tracking error signal detection by the push-pull method, a problem has been pointed out for many years. This is a problem that a false tracking error signal (hereinafter referred to as “offset”) is generated because, when the objective lens is moved in connection with the tracking operation (hereinafter referred to as “lens shift”), the optical spot on the photodetector also moves in accordance with the movement.
One of methods proposed as a method for preventing this is the differential push-pull method, which is disclosed by the second embodiment of JP-A No. 94246/1986. In the differential push-pull method, two sub spots are arranged, on both sides of a focused spot on the disk plane, at a distance shifted from the focused spot in a radial direction by one-half the guide groove pitch, and the differential signal between the push-pull signal by the central focused spot (hereinafter referred to as a “main spot”) and the push-pull signals of the sub spots is used as the tracking error signal. The offset associated with the lens shift occurs in the same polarity for the main spot and for the sub spots, whereas the tracking error signal occurs in the reverse polarity for the main spot and for the sub spots because the main spot and the sub spots are arranged with their positions being shifted by one-half the guide groove pitch. Therefore, by obtaining the differential signal of these error signals, a component of the tracking error signal is enhanced and the offset is canceled out. A diffraction grating that generally has a row of linear unit gratings is used to form the sub spots, and sub-spot positions on the disk are adjusted by rotation adjustment of the grating.
Another method for preventing occurrence of the offset by the lens shift in the push-pull method is a method that uses the polarization-dependent diffraction grating. The polarization-dependent diffraction grating is a diffraction grating made of a birefringent material such as anisotropic optical crystals and liquid crystals, and it has a function of acting as a diffraction grating for one linear polarization whereas it has a function of generating no diffracted light for linear polarization perpendicular to that linear polarization because phase difference caused by the grating becomes an integral multiple of the wavelength of the light. If this component is integrated with a quarter-wave plate in one body and linearly polarized light from which no diffracted light is produced while passing through the component is entered into the integrated body, the incident light passes through the quarter-wave plate and is transformed to circularly polarized light without being diffracted by the polarization-dependent diffraction grating. When this light is reflected by an optical disk etc. and reenters the polarization-dependent diffraction grating, diffraction effect takes place in the polarization-dependent diffraction grating because after first passing through the quarter-wave plate the light is transformed into linearly polarized light whose polarization direction is perpendicular to that of the linearly polarized light at the time of entering the integrated body for the first time. This arrangement can provide a diffraction grating that does not diffract the light in an outward travel and diffracts the light only in a homeward travel. Based on this principle, a grating has been formed to diffract two regions of light that are to be separated and detected by the push-pull method in different directions, and is moved together with the objective lens as one body for the tracking. If light fluxes after being separated by the polarization-dependent diffraction grating are detected by the photodetectors whose light detecting regions are large enough to receive the flux without failing to receive any portion thereof even when the lens shift is done, there is effectively no displacement of the received light flux with respect to a dividing line of the light flux. Thereby, the occurrence of the offset can be mitigated. However, this method is incomplete in suppressing the offset as compared with the differential push-pull method. The occurrence of the offset by the lens shift is affected not only by the movement of the dividing line for dividing the light flux but also by displacement between the optical axis of the objective lens and the center of the intensity distribution caused by a fact that the intensity distribution of the out-going light from the semiconductor laser has a Gaussian distribution. In the method in which the polarization-dependent diffraction grating is moved together with the objective lens in one body, the effect by the movement of the dividing line is dissolved, but the influence by the displacement of the center of the intensity distribution remains. Therefore, this method may turn to be insufficient in cases where the push-pull signal amplitude is small, to be more precise, a case where the guide groove pitch is narrow as compared with the focused spot, a case where the guide groove is shallow, a case where the guide groove width is narrow as compared with the guide groove pitch, etc., that is, cases where the remaining offset becomes notably large.
In the twin beam method, two sub spots are formed on the both sides of the focused spot on the disk, and they are arranged at positions shifted from the main focused spot in a radial direction by one-quarter the guide groove pitch. The tracking error signal is obtained by detecting variation in the total reflected light quantity of each sub spot caused by tracking error between the guide groove on the disk and the sub spots and finding a difference signal between the total reflected light quantities. To form the sub spots, normally a diffraction grating that has a row of linear unit gratings is used, and the sub-spot positions on the disk are adjusted by rotating the grating. Since in the twin beam method the tracking error is obtained by detecting the variations in the total reflected light quantities, its detection sensitivity becomes highest when the guide groove depth is equal to an optical depth of ¼ wavelength where the total reflected light quantity exhibits a largest change. Further, since the signal is obtained not from the distribution but from the total light quantity, there does not occur offset that is caused by the lens shift and may become a problem in the push-pull method. Further, since this method requires that the reflected light quantity when the spot is on the guide groove differ from the reflected light quantity when the spot is between the guide grooves, if the guide groove width and a spacing between the guide grooves are the same in dimension, the signal cannot be detected theoretically.
The differential phase detection method is a method that is used in DVD-ROM, in which detected is the phase difference between two signal waveforms each of which is obtained by adding the received light quantities falling on diagonally located two regions of the four-segment regions of the photodetector that is produced by an interference effect of the diffracted light fluxes caused by the pit sequence and is proportional to the tracking error on time base. The phase difference becomes largest when the optical depth of the pit is equal to ¼ wavelength, which agrees with an optical depth at which a reproduced signal amplitude becomes largest. When the optical depth of the pit is ¼ wavelength, the offset by the lens shift does not occur theoretically as well.
As mentioned in the foregoing, in terms of features of the tracking methods, suitable tracking methods are chosen for respective classifications of disks.
That is, in DVD-ROM, since the recording density is high and the optical depth of the pit that were previously recorded is set to ¼ wavelength in order to make the signal percentage modulation large, the differential phase detection method is suitable. It is thought possible to apply the twin beam method. However, the differential phase detection method can perform detection with only one spot, and hence the differential phase detection method is chosen. The push-pull method cannot be applied because theoretically the signal amplitude reduces to zero.
In CD-ROM, the phase depth of the pit is set to ⅙ wavelength in order that the tracking error signal can be detected even by the push-pull method in specifications. Therefore, considering the influence of the lens shift upon the offset, the twin beam method is suitable. However, even in the differential phase detection method, a method where by the offset is mitigated by improvement of signal processing has been developed.
In DVD-RAM, the guide groove width is set identical to the spacing between the guide grooves, and the land and groove recording system whereby information is recorded both on the guide groove and between the guide grooves is used. For this reason, the twin beam method is not used therefor, and a method that uses the polarization-dependent diffraction grating is commonly used because the guide groove pitch is wider than that of the under-mentioned DVD-R etc. and can provide a sufficiently large push-pull signal. Although the differential push-pull method has a larger effect of suppressing the offset, the method that uses the polarization-dependent diffraction grating that uses only one beam has been chosen in terms of securing a sufficient margin of light intensity for the recording.
In DVD-R, DVD-RW, CD-R, and CD-RW, the guide groove pitches are narrower than that of the land groove method and information is recorded on the guide grooves whose phase depths are shallow. For this reason, the method that uses the polarization-dependent diffraction grating reduces the effect of suppressing the offset. Further, these optical disks are featured especially by being a low price, it is often the case that the differential push-pull method is chosen rather than the method that uses a somewhat costly polarization-dependent diffraction grating. However, since generally these disks are intended to be equivalent to DVD-ROM and CD-ROM when being reproduced, the same tracking method as DVD-ROM and CD-ROM is applicable in principle when being reproduced.
The optical heads for recording and reproducing the respective optical disks are required to be miniaturized in connection with miniaturization and thinner design of the apparatuses. For example, a hologram laser module capable of reproducing both DVD and CD is disclosed by JP-A No. 120568/1997 (the first conventional example). The hologram laser module is such that a semiconductor laser, a photodetector, and a hologram for guiding reflected light from the optical disk to the photodetector are modularized into one body. In this conventional example, the twin beam method is employed for both DVD and CD (in this conventional example, being referred to as “three beam method”). On the other hand, an optical head that uses a modularized part that is supposed to be used for recording on DVD is disclosed by JP-A No. 126297/2001 (the second conventional example). In this example, a semiconductor laser and a photodetector are modularized into one body, but a hologram for guiding the reflected light from the disk to the photodetector are integrated with the objective lens, being separated from the module. This configuration is adopted because the tracking of recording optical disks, such as DVD-RAM, is assumed.
Further, in the DVD Multi drive described previously, in order to perform tracking of the above-mentioned all classifications of disks, it is very likely that a method in which the polarization-dependent diffraction grating is moved together with the objective lens as one body will be used. In this case, although the offset by the lens shift is not removed completely and its influence remains for DVD whose tracking error signal amplitude is small or the like, this incompleteness is solved by margin allocation design of the whole system.
In the above-mentioned first conventional example, since the twin beam method is employed to execute the tracking control of DVD and CD, no tracking error signal can be obtained for DVD-RAM whose guide groove width is equal to the spacing between the guide grooves. Further, in DVD-R/RW etc. whose guide groove is shallow, the tracking error signal amplitude becomes extremely small. For these reasons, this conventional example is not suitable for recording disks, and hence cannot be applied to the DVD Multi drive.
In the above-mentioned second conventional example, since the polarization-dependent diffraction grating is used to execute the tracking control by the push-pull method, the offset by the lens shift occurs in DVD-R/RW. Further, since it is necessary to mount the polarization-dependent diffraction grating on the objective lens to be an integrated combination, its weight will be a problem in enhancing the speed in the future. Moreover, since the polarization-dependent diffraction grating is independent from the module wherein the semiconductor laser and the photodetector were integrated, it is likely that displacement between the polarization-dependent diffraction grating and the module due to temperature variation and/or change over time will occur. The occurrence of the displacement deteriorates accuracy of focus control of the focused spot and of tracking control, causing degradation of the reproduced signal.
As a method whereby a hologram (diffraction grating) for branching off detected light without generating the offset by the lens shift even in DVD-RAM and DVD-RW can be integrated with the semiconductor laser and the photodetector, there is the differential push-pull method. However, it is necessary to arrange the sub spots on the disk at positions shifted from the main spot in a radial direction by one-half the guide groove pitch, but the guide groove pitch is almost two times different between DVD-RAM and DV-RW. DVD-RAM is based on the land groove system whereby information is recorded both on the guide groove and between the guide grooves, whereas DVD-RW etc. are based on a method whereby information is recorded only on the guide groove. Since the both have almost the same recording density, the guide groove pitch of DVD-RAM is two times wider than that of DVD-RW etc. Therefore, positions of the sub spots that are necessary for the differential push-pull method are different for DVD-RAM and for DVD-RW etc., and hence the differential push-pull method cannot be compatible for the two kinds of systems.